The present invention relates to a printing method for a printer containing a transport system for a recording medium, and a printhead with a plurality of print units, each of which is capable of printing a pixel line when the printhead is scanned over the recording medium, wherein a failure compensation unit controls the print operation and compensates for a failure of a print unit.
An example of a printer of this type is disclosed in EP-A-0 981 105, which relates to an ink jet printer. In the printhead of this printer, the print units are formed by ink jet nozzles which are arranged in a linear array extending in the direction in which the recording medium is transported. Thus, when the printhead is scanned over the recording medium, a swath of an image can be printed, and the number of pixel lines in the swath corresponds essentially to the number of nozzles present in the printhead.
Such a printer can generally operate in different print modes. In a single-pass mode, each nozzle of the printhead prints the complete image information of a pixel line during a stroke in which the printhead is moved over the paper. Then, the paper is transported over the width of the swath that has been printed, and the next swath is printed in a return stroke of the printhead. As an alternative, a two-pass mode may be applied, in which each nozzle prints only every second pixel of the corresponding line during the first stroke, and the missing pixels are inserted in the return stroke of the printhead. In this mode, the paper may be transported in steps which correspond to only half the length of the nozzle array. Then, one half of the nozzles will be used for printing every second pixel of a new swath, whereas the other half of the nozzles is used for inserting the missing pixels in the swath that had been printed in the previous stroke. As a result, two different nozzles will be involved in printing all the pixels of a given pixel line.
If a nozzle of the printhead becomes clogged or fails for any other reason, the pixels that would have been printed with the inoperative nozzle will be missing in the printed image, and the image quality will be impaired. A variety of failure compensation strategies are known for avoiding or mitigating this undesirable effect.
For example, the above-mentioned document proposes a compensation strategy which employs the two-pass mode. Here, the job of the inoperative nozzle is taken over by the nozzle which is normally utilized only for inserting the missing pixels. Of course, if the scanning speed of the printhead is not reduced, this requires that the nozzle that is used for failure compensation is capable of printing pixels with twice the normal frequency.
EP-A-1 060 896 discloses a failure compensation strategy which is also applicable in a single-pass mode. When, in the event of breakdown of a nozzle, a specific pixel should but cannot be printed with the inoperative nozzle, this pixel is transferred to an addressable position in the vicinity of the designated pixel position, so that it can be printed with another nozzle. This strategy helps to prevent loss of information but will not fully compensate the nozzle failure and is in many cases sufficient for suppressing the visual effect of the nozzle failure below acceptable limits.
Another known failure compensation strategy is particularly applicable to the case where a breakdown of a nozzle or, more generally, a print unit occurs near the end of the nozzle array. Then, an end section of the nozzle array, which section includes the inoperative nozzle, is cut-off, i.e. the nozzles of this section are disabled. As a result, the usable length of the nozzle array is somewhat reduced, and the swath of the image that is printed in a single stroke is reduced in width. By adapting the transport width of the recording medium to the reduced width of the swath, a defect-free image can be printed, although at the cost of productivity.
In general, unless redundant nozzles are present in the printhead, failure compensation involves a tradeoff between productivity and image quality.